Force limiting mechanisms of the general type contemplated herein normally comprise a worm shaft meshed with a worm gear. The shaft is mounted for rotary movement about its axis, and also for a linear movement along its axis. Biasing means, typically comprising one or more springs, serves to maintain the axial position of the shaft. As the load transmitted by the drive mechanism increases, so the biasing means is progressively overcome, and the lateral movement of the worm shaft may serve to actuate a drive control function such as a brake or clutch directly, or indirectly through appropriate switch mechanism.
Drive mechanisms of this type are relatively complex, and may require, for example, two part shafts which are telescopically slidable along splines which transmit the rotary drive force therebetween, and tend to be unnecessarily complex. They are, moreover, relatively restrictive in the type of motion that may be transmitted therethrough, this being essentially rotary motion.